Afar magazine suggests that if your flight is asking for volunteers to be bumped, ask to get an upgrade to business class. Not sure how well that would work.
They also mention that many of the bidding to upgrade programs run through a site called Plusgrade.com. My two recent experiences with flight showed me that different airlines handle upgrades differently, United was very gentle and softspoken about the option and Delta practically hung onto my shirt collar and begged me to do it.
I recall decades ago, when I went to ask the gate agent a question, she asked if we would be interested in taking a later flight since the first one, which we had a reservation on, was overbooked. I asked if first class was available, she said it was, so in addition to receiving a voucher, we were able to sit in first class. This was a thrill for budget-minded me back when I was in my thirties. Nowadays, I would not bother giving up my seat for a voucher and upgrade on a later flight, as I would prefer to get to my destination on time.
